HYMENOCARIS VERMI CAUDA. 75 



Hymenocaris vermicauda has its carapace folded or bent along the back, so as 

 to form an overarching carapace (as in Nehalia) with two symmetrical, suboval, 

 valve-like sides, somewhat resembling saddle-flaps, not so deep iu front as behind, 

 obliquely rounded or semi-elliptical below, and with a very slightly convex dorsal 

 line.^ The curvature of the ventral edge varies in fulness and in obliquity with 

 individuals, and is nearly always modified by the pressure to which the shale or 

 mudstone containing the fossils has been subjected. The specimens are all flattened 

 laterally ; some are lengthened, and some shortened, according to their position 

 relative to the direction of the squeeze; and nearly all are crumpled or " plaited"" 

 with parallel foldings, coarse or fine, at right angles to the line of pressure. 



Some of the best preserved individuals measure ^o i^ch (21 mm.), others 

 1 inch (23 mm.), and others (imperfect otherwise) even more, along the back line. 

 Those with the first two measurements are ^o iiich (19 mm.) in height ; and their 

 angular length (from antero-dorsal to postero-ventral point) is I-^q inches 

 (56 mm.). Many smaller individuals occur. 



The carapace was thin (hence the name ==" membranous") and apparently 

 smooth. No definite structure has been obsei^ved, but Salter noted " short wavy 

 lines " on the cai'apace and the abdominal segments (' Mem. Geol. Survey,' vol. iii, 

 p. 294), and a marginal furrow along the posterior border of the valves (p. 293). 



Owing to the compressed and plaited condition of the schistose matrix, it is 

 difficult to define the original outline of the ends of the carapace. The fig. 4 in 

 pi. ii, ' Mem. Geol. Surv.,' vol. iii, is a restoration, and its truncate anterior end is 

 a very doubtful feature. The outline given of a specimen shown in fig. 3 (loc. 

 cit.) is not supported by the specimen itself.^ The front angle, though often 

 modified or suppressed by the imperfect cleavage of the squeezed mudstone, is 

 sometimes perfect enough to show that it was much sharper than in the fig. 4 

 referred to above, in which the truncation is probably due to fracture of one of 

 the specimens used in making this restoration of the animal. The posterior 

 margin appears to have sloped downwards and outwards, with a bold ventral 

 curve, but usually without the sinuous (ogee) bend under the dorsal angle which 

 Ceratiocaris generally exhibits. 



The relative position of carapace and body-segments has been subjected to 

 much interference, between the death and the imbedment of the specimens, from 

 the decomposition of the soft parts or connecting tissues and the shifting of the 



* Much like the carapace-valves of Ceratiocaris Pardo'eana in shape (Monogr., part i, 18S8, pi. v, 

 figs. 1 and 2. 



2 Salter, ' Quart. Journ. Geol. Soc.,' vol. x, 1854, p. 209 ; and ' Mem. Geol. Surv.,' iii, 1866, 

 p. 247, note. 



3 The two small spikes figured in the same illustration are illusory, as are also the three needle- 

 like projections in the woodcut in ' Siluria,' 1854, 1859, and 1867, though often copied in text- 

 books. 



